1. Field of the Invention
The present invention relates to a surface emitting laser.
2. Description of the Related Art
Vertical-cavity surface emitting lasers (VCSEL) are used as light sources of exposure optical systems included in electrophotographic image forming apparatuses.
A surface emitting laser used as a light source of an image forming apparatus is desired to oscillate in a single transverse mode.
Japanese Patent Laid-Open No. 2001-284722 discloses a technique of causing a surface emitting laser to oscillate in a single transverse mode by providing a relief structure, i.e., a stepped structure, on the output surface of the surface emitting laser.
That is, the stepped structure is configured such that the reflectance of a portion thereof extending in central part of an emission area is higher than the reflectance of a portion thereof extending in outer part of the emission area. In the fundamental transverse mode, the distribution of light intensity is concentrated in the central part, compared with distributions of light intensity in higher-order transverse modes. Furthermore, the presence of the stepped structure increases the reflectance in the central part. Therefore, light oscillation in the fundamental transverse mode can be selectively caused. Thus, a single-transverse-mode surface emitting laser is provided.
Such a surface emitting laser may include an oxidized confinement structure that defines the emission area of an active layer. The oxidized confinement structure is obtained by oxidizing a semiconductor layer from the sidewall of a mesa structure including the semiconductor layer toward the center of the structure. The oxidized confinement structure has a lower refractive index in an oxidized and thus insulating region (outer region) thereof than in a semiconductor region (central region) thereof, thereby forming a refractive-index guiding structure. Consequently, the oxidized confinement structure determines the profile of the resonance mode.
In general, the fundamental transverse mode of a cavity of a surface emitting laser forms a function curve representing a symmetrical intensity distribution. The intensity distribution in the fundamental transverse mode substantially conforms to the light intensity distribution in the near field. The light intensity distribution in the near field, i.e., the near field pattern (NFP), is a function curve representing a symmetrical intensity distribution. The phase distribution (wavefront) of light in the near field is even, forming a plane perpendicular to the direction of resonance.
The light distribution (electric-field complex amplitude) in the far field is obtained by Fourier transformation of the light distribution (electric-field complex amplitude) in the near field. If the light intensity distribution is symmetrical and the phase distribution is even in the near field, the phase distribution in the far field is even. The near field pattern (NFP) refers to an electric-field intensity distribution in the near field (a plane immediately after the light output surface). A far field pattern (FFP) refers to an electric-field intensity distribution in a spherical plane centered on the light source with a radius ∞.
In the surface emitting laser including a stepped structure disclosed by Japanese Patent Laid-Open No. 2001-284722, the stepped structure has different optical path lengths in the central part and the outer part. Accordingly, the near-field wavefront is modulated by the stepped structure, making the far-field phase distribution not even. Consequently, the far-field wavefront is shifted from the reference spherical surface centered on the light source, resulting in wavefront aberration.
Particularly, when a surface emitting laser is employed as a light source of an image forming apparatus, the wavefront in the entrance pupil of the exposure optical system included in the image forming apparatus is desired to conform to the reference spherical surface. If the wavefront in the entrance pupil does not conform to the reference spherical surface, the imaging position is shifted from the conjugate point, with respect to the light source, in the image plane at the time of focusing or defocusing.